Charge-coupled device (CCD) image sensors typically include an array of photosensitive areas that collect charge carriers in response to illumination. The collected charge is subsequently transferred from the array of photosensitive areas and converted to a voltage from which an image may be reconstructed by associated circuitry. FIG. 1 depicts a conventional interline CCD image sensor 100 that contains an array of photosensitive areas 110 (each of which may include or consist essentially of a photodiode, photodetector, photocapacitor, or photoconductor) arranged in columns. A vertical CCD (VCCD) 120 is disposed next to each column of photosensitive areas 110, and the VCCDs 120 are connected to a horizontal CCD (HCCD) 130.
As shown schematically in FIG. 1 (for only one column of photosensitive areas 110, for clarity), following an exposure period, charge is transferred from the photosensitive areas 110 into the VCCDs 120, which subsequently shift the charge, row-by-row in parallel, into the HCCD 130. The HCCD then transfers the pixel charge serially to output circuitry, e.g., an output charge-sensing amplifier 140. The resulting data is then typically digitized, and the digitized image is displayed on a display or stored in a storage unit.
In order to obtain proper image contrast, brightness, and color levels, a signal reference is selected in order to properly calibrate the black level of the digitized image. In principle, extra horizontal clock cycles could be added to the timing sequence and utilized to produce the black reference; however, such techniques may not be advisable because, regardless of light level, both the photosensitive areas 110 and VCCDs 120 generate a thermal signal, known as dark current, that acts to brighten the apparent “darkness” of black levels in these regions. Since the mere use of additional HCCD cycles does not correct for dark currents in the photosensitive areas 110 and VCCDs 120, black regions in the image will appear gray due to this improper calibration level (or improper “black clamp”).
FIG. 2 depicts a common technique for correcting the black level for dark currents (or “dark signal”) for an interline CCD image sensor. As shown, an interline CCD image sensor 200 includes a light-sensitive image array 210 surrounded by regions 220, 230, 240, 250 of dark reference pixels. The light-sensitive image array 210 includes optically active pixels utilized to acquire images as detailed above. Charge from the optically active pixels and the dark reference pixels in regions 220, 230, 240, 250 is transferred into an HCCD 260, and the charge corresponding to each row is read out to a charge-sensing amplifier 270.
As indicated by the shading on FIG. 2, a metal light shield is typically used to prevent light from entering the dark reference pixel regions 220, 230, 240, 250. Typical materials for the light shield include aluminum, copper, tungsten, or TiW. In principle, only one of the four regions 220, 230, 240, 250 is necessary to provide a black clamp. In practice, however, regions 240, 250 are most often used as the black-level reference, and the black level is set on a row-by-row basis to minimize row noise in the system electronics.
In theory the dark signal from the regions of dark reference pixels 240, 250 should be the same as from the light-sensitive image array 210. However, in practice the dark signal may vary substantially between the two regions. For example, often a solid metal sheet will be utilized to block incoming light in dark reference regions 240, 250. In such situations, black regions in the image area will appear blacker than the black level set according to regions 240, 250 if, for reasons explained below, the dark signal in the dark reference regions 240, 250 exceeds the dark signal from the image array 210. This difference in dark signal between the image area and dark reference region is generally referred to as a “dark step,” and results in an improper black clamp.
FIGS. 3A and 3B illustrate the origin of the dark step. FIG. 3A depicts a typical cross-section through image sensor 100 along the line A-A′ in the optically active region, and FIG. 3B depicts a similar cross-section within the dark reference region of the image sensor (e.g., within region 240 or 250 of image sensor 200). As shown, the image sensor is typically fabricated utilizing a semiconductor structure 300. Semiconductor structure 300 typically includes an n-type substrate 305 (e.g., a silicon substrate), a p-type well 310 that acts as a vertical overflow drain (as known in the art), an n-type photodetector 315 in which charge (i.e., “photocharge”) induced by incident light accumulates, and a p-type pinning layer 320 that reduces photodetector dark current. The image sensor also includes an n-type VCCD 325 and, surrounding the VCCD 325, a p-type transfer gate region 330 and two p-type channel stop regions 335, 340.
Above the semiconductor structure are thin dielectric layers 345, 350, which typically include or consist essentially of SiO2, HfO2, or SiO2/Si3N4/SiO2. For some CCD image sensors, the dielectric layer 345 over the photodetector 315 and the dielectric layer 350 over the VCCD 325 are portions of the same layer, but in other devices they are different and distinct layers. The image sensor also includes a gate 355 that is typically formed of polysilicon, indium tin oxide (ITO), or metal. Application of a large voltage to the gate 355 transfers an amount of photocharge (or a “charge packet”) from the photodetector 315 to the VCCD 325. The multiple gates over the VCCD register 120 (as shown in FIG. 1) are then clocked sequentially to move multiple charge packets row-by-row into the HCCD 130. As described in U.S. Pat. No. 5,250,825, the entire disclosure of which is incorporated by reference herein, a metal light shield 360 covers the VCCD 325. The light shield 360 prevents entry of light into the VCCD 325, as such light generates undesired extra charge carriers during the time necessary to read out the image from the VCCD 325. The light shield 360 is typically electrically isolated from the transfer gate 355 by dielectric layers 365, 370, and covers only the VCCDs 325, so as not to prevent entry of light into photodetectors 315 during image acquisition.
A major source of dark current in VCCD 325 is unterminated silicon bonds at the silicon/dielectric interface above the VCCD 325. Typically a sintering step (e.g., an anneal in a hydrogen ambient) is included late in the manufacturing process in order to passivate such unterminated atomic bonds. A typical sinter step may be performed in the 350° C. to 500° C. range for 30 minutes to 2 hours. Hydrogen in the sinter environment (or from sources within the device itself) diffuses throughout the device during the sinter process. Hydrogen reaching the silicon/dielectric interface is often captured by the unterminated bonds, thus passivating the silicon surface and reducing the VCCD dark current dramatically.
As shown in FIG. 3B, in the dark reference region 240, 250, a light shield 375 is typically a solid metal sheet that covers not only VCCDs 325 but also photodetectors 315 in order to prevent incoming light from affecting the measured dark signal. However, the diffusion of hydrogen is blocked by most metals. Thus, in order to reach the silicon/dielectric interface in the dark reference regions 240, 250, hydrogen must first diffuse around (rather than through) the dark reference light shield 375. As a result, less hydrogen reaches the silicon/dielectric interface, fewer unterminated bonds are passivated in dark reference regions 240, 250, and the deleterious dark step occurs. Thus, there is a need for CCD image sensor designs enabling passivation of unterminated bonds in the dark reference region while preventing light from impinging thereon, thereby enabling improved calibration of dark levels in images acquired by the sensor.